1. Field of the Invention
This invention relates to a photo-semiconductor device having a function of transmitting/receiving a high frequency electric signal mainly in a frequency range between milli-meter and tera-hertz waves and a method of manufacturing the same.
2. Related Background Art
So-called non-destructive sensing technologies using electromagnetic waves of a frequency range between milli-meter and tera-hertz waves (30 GHz to 30 THz) have been developed in recent years. Technologies using electromagnetic waves of such a frequency range include those for imaging, using fluoroscopic inspection apparatus that are safe and can replace X-ray apparatus, those for examining the internally bonded states of substances by determining absorption spectra and complex dielectric constants, those for analyzing biomolecules and those for evaluating carrier densities and carrier mobilities. Position sensing technologies have also been developed for anti-collision radars using 70 GHz band waves, or milli-meter waves.
For example, Japanese Patent Application Laid-Open No. 2002-98634 discloses a two-dimensional imaging apparatus designed to spatially expand a pulsed beam of light in a tera-hertz frequency range, irradiating it onto a specimen and observing a two-dimensional fluoroscopic image in a time domain. FIG. 9 of the accompanying drawings is a diagram schematically illustrating the disclosed apparatus. The pulsed visible beam of light emitted from a light source 121 is branched by a half minor 128, and one of the branched light beams, or light beam 121b, is irradiated onto a tera-hertz light source 122, which is referred to as a photoconductive switch and is adapted to convert a light pulse into an electromagnetic pulse that corresponds to the envelop of the light pulse. The generated tera-hertz beam of light is transmitted through a specimen 125 by means of an optical system 123 and converged to a tera-hertz beam detector 126. On the other hand, the other output 121a is delayed by a movable minor 124 and irradiated onto the tera-hertz beam detector 126. The time domain gauging can be performed by gating control that allows the reception signal of the tera-hertz beam to be taken out only at the timing of irradiation of the light pulse. A photoconductive switch having the structure same as that of the tera-hertz beam generator 122 is used as the tera-hertz beam detector 126.
A photoconductive switch element disclosed in Japanese Patent Application Laid-Open No. 10-104171 and realized by arranging an antenna, which operates also as an electrode, on a photoconductive film that is formed on a substrate can suitably be used to generate and detect a tera-hertz beam of light. FIG. 10 of the accompanying drawings schematically illustrates such a photoconductive switch element. As shown in FIG. 10, the substrate 130 of the photoconductive switch element is a silicon-on-sapphire structure that is radiation-treated. In other words, a silicon film that is a photoconductive material is formed on a sapphire substrate. LT-GaAs grown on a GaAs substrate at low temperature is often used as photoconductive film in place of the silicon-on-sapphire structure. The dipole antenna 138 formed on the surface comprises a pair of dipole feeder lines 138a and 138b and a pair of dipole arm sections 139a and 139b. A tera-hertz (THz) pulse is generated when the light pulse is converged to the gap 133 and a voltage is applied to the gap 133 so that it is possible to detect the THz pulse by detecting a photocurrent by means of current amplifier 134 without applying a voltage. Substrate lens 136 takes a role of coupling an electromagnetic wave from a slab mode (substrate mode) of being confined to substrate 130 to a radiation mode of being radiated to free space and also a role of controlling the radiation angle of electromagnetic propagation mode in space.
On the other hand, a wavelength-variable tera-hertz beam of light showing a high degree of spectrum purity is required for producing a spectral effect in a wavelength domain instead of gauging in a time domain. Applied Physics Letters 70(5), pp. 559-567 (1997) describes an arrangement for generating a wavelength-variable tera-hertz beam of CW light by generating the difference frequency of two laser beams by mixing that can be used for such an application. An element similar to the one illustrated in FIG. 10, or an arrangement for generating a tera-hertz beam of light by means of a photoconductive switch by applying a voltage between two electric conductors formed on the surface of a compound semiconductor and irradiating a mixing beam of light to the gap between the electric conductors, is used as means for converting a mixing beam of light into a tera-hertz beam of light.
When realizing a photoconductive switch as described above, it has not been possible to select a material and a profile for the substrate freely, because it has been required to form a photoconductive film that is a semiconductor layer on a given substrate in order to produce desired characteristics.
For example, when adding a lens in order to reduce the electromagnetic wave components that do not radiate into free space in a substrate mode, it is desirable to form a substrate and a lens so as to make them show a substantially same dielectric constant from the viewpoint of the efficiency of taking out an electromagnetic wave. In other words, it is ideally desirable to form a substrate and a lens by means of a same material. Normally, electrically highly resistant Si that shows little wavelength dispersion and little absorption loss in the tera-hertz region works as an excellent lens material. However, it has not been possible to use such an ideal structure because it is not possible to form a photoconductive film that operates satisfactorily on an Si substrate. In other words, reflection of light occurs at the interface of the substrate lens and the substrate of the photoconductive element due to the difference of dielectric constant between them because two different materials have to be used for them.
While the reflectivity of the thin film at the interface of the photoconductive element and free space can be reduced by using a material that shows a low dielectric constant in order to reduce the substrate mode. However, a photoconductive film cannot be formed by using a glass substrate or a plastic substrate showing a low dielectric constant.
A laser beam emitted from a laser has to be converged to a very small spot when it is irradiated onto a photoconductive element. A high output power THz generator can be realized by raising the output power of the laser if the substrate shows a high thermal conductivity. Materials that can be used for a substrate showing such a high thermal conductivity include Si, AlN and SiC because they show a thermal conductivity more than three times of the thermal conductivity of GaAs and sapphire. However, it is not possible to directly form a photoconductive film on any of them.
On the other hand, Si substrates, glass substrates and plastic substrates are excellent substrates in terms of cost and safety for forming hybrid modules by integrally combining photoconductive elements, THz waveform transmission paths and antennas.